Coal pulverizers which prepare coal for firing boilers are needed to reduce coal to very fine grades with a substantial degree of reliability. It is important to produce coal which both burns efficiently and cleanly. Finer grades of coal are necessary to support boiler firing techniques which suppress nitrous oxide production. Finer coal burns more completely and produces less smog.
Various centrifugal type pulverizer machines operate on the concept of material being fed through an axial feed tube into the center of a high speed rotor with vanes that expel the coal or process material at high velocities. The material dissipates large amounts of energy on material banked walls or anvils causing size reduction. The Spokane Model 120 and the Barmac Duopactor are in this class. However, none of these devices prevent wear of metal parts due to collisions of the material.
Ways have been found to cause material to collide with itself, thus sparing wear on metal parts. Santos, in U.S. Pat. No. 4,366,929 describes a machine in which material is made to change direction rapidly and collide with other material. Weinert, in U.S. Pat. No. 4,340,616, protects surfaces with a sufficient layer of material held by magnetic attraction.
Brown, et al in U.S. Pat. No. 5,275,631 describes a coal pulverizer in the form of rotating rings within which material banks up against the inside walls and then is thrown out against counter-rotating inverted rings which are self-protected in the same way, combined with aerodynamic and electrostatic separators. The contents of U.S. Pat. No. 5,275,631 are incorporated herein by reference thereto.
There are many cage type mills which incorporate impacting members on counter-rotating rotors. Some of these depend on impacts of particles thrown between counter-rotating elements, with additive velocities at impact. Other counter-rotating designs are aimed at producing interactive air movements for churning particles against one-another with little contact with wearable components. The latter group have been commercially successful only in limited throughput capacities up to 5 tons per hour. Their design principles have not scaled up efficiently, rotating at over 3000 RPM in order to move mostly air, which in turn moves particles.
The Nickel U.S. Pat. No. 5,009,371, issued in 1991, describes a disintegration chamber in which vortex zones of gas/solids mixture are formed within annular chambers defined by the front and rear edges of opposed blades. In some devices of this type as much as 60 percent of the reduction apparently takes place without the particles contacting the blades or impact members.
Earlier rotary disintegrators have depended solely on contact with rotary impact members. The Hint U.S. Pat. No. 3,497,144 is of this type, using a particular configuration of rotor bars to impact particulate material. Noe, as early as 1968, in U.S. Pat. No. 3,411,724, describes a cage type disintegrator in which blades are angled 20 to 30 degrees and are "substantially concave" on the active surface in order to retain process material for wear resistance. Durek, in 1983, in U.S. Pat. No. 4,406,409, describes a machine with four or more rows of concave scoops, angled at 20 to 30 degrees for optimal impacting and particle retention. Mushcenborn, in 1985, describes angled impact elements with a "trailing profile of streamlined cross section" which is meant to eliminate "cavitation phenomena and hence reduces vortex formation and turbulences."
The use of cage mills or other rotary disintegrators to produce the fine, superfine or ultrafine grades of pulverized coal, however, generally has not been done efficiently at the high production rates required for feeding utility and large industrial boilers, that is 20 to 75 tons per hour. There is a need for a device which can both protect the metal parts from wear and can maximize efficient reduction of the process material at high rates to finer grades.